Effects of burn-in on power supply reliability

Users of power supply products call for increasingly higher levels of reliability and performance. Although the suppliers of individual components can confidently provide impressive life and reliability data, the compound effect on overall reliability when a large number of individual components are combined in a module such as a power supply can be significant. Perhaps more important for product reliability is the quality and repeatability of the assembly process. Solder joints, connectors and mechanical fixings are all potential origins for product failure. In use, operating temperature and other environmental factors also affect the life and reliability of a power supply.

Burn-in and various other forms of life and stress testing help provide the data to enable power supply manufacturers to continually improve the reliability of their products. When analysed correctly and fed back into the design and assembly process, the accumulated data can be used to optimise the test and burn-in process.

The burn-in processThe purpose of the burn-in process for power supplies which have passed initial manufacturing test is to weed out "infant mortalities" as seen in the first portion of the well-known "bathtub curve" of failure rate versus operating time (figure 1). These early life failures may be due to latent intrinsic faults within bought-in components, marginal workmanship errors or latent faults induced in components by inappropriate handling such as ESD damage. Note that there are no absolutes in the world of reliability testing; only probabilities and confidence levels for large populations so there is never a guarantee that all infant mortalities are caught by the burn-in process.

Over many years the conventional approach to power supply burn-in has involved running products at an elevated temperature, often the maximum-rated operating temperature, where the rate of appearance of latent defects is assumed to be accelerated. The supplies are run under full load with power cycling and the input voltage is run at either maximum or minimum to provide either maximum voltage stress or maximum current stress, depending on the design topology.

Care in the choice of conditions is necessary because some components in some topologies can see more stress at light loads, such as snubber networks in variable-frequency converters. Some ingenuity can also be applied. For example, if a product is intended to operate normally with forced air, it could be run in still air at light load and still achieve comparable temperature stress levels of the hottest components. However, without the "heat spreading" effect of the forced air, other components might see very little stress under these conditions.

A technique sometimes used by Murata Power Solutions, dependent on the product topology, is to burn-in products into outputs cycled between short and open circuit. This can apply an appropriate current-stress level while exercising the inbuilt protection circuitry on short circuit and imposing a high-voltage stress level to many components on open circuit. There is a major benefit in the fact that the power in the short- or open-circuit load is theoretically zero, although practically the short might be applied by a MOSFET dissipating a few watts.

This method alleviates the real problem of energy wasted in burn-in loads. However, some types of component stresses are not applied with this method because the overall power supplied by the unit might be low and therefore self-heating may be low. An elevated ambient temperature will compensate for this in part, perhaps using the waste heat from the burn-in loads.

Some product topologies are not suitable for this burn-in method, such as those that have a poorly defined or strongly re-entrant short-circuit current characteristic. For example, if on a "hard" short circuit the output current reduces to much less than the rated maximum output current or if the supply enters a "hiccup" mode, the level of burn-in stress may be too low to be effective.

The decision on burn-in configuration is made jointly between design and reliability/quality engineers to ensure optimised screening. Data logging and analysis of the units under test is important for determining whether and when a failure has occurred. If all failures occur in the first few minutes of a 48-hr burn-in sequence, there would be good reason to shorten the time and increase throughput while saving energy.

A comprehensive test after burn-in is necessary to ensure that products are fully functional. This can also show whether there are any intermittent problems. Understanding and using burn-in data to modify product design and manufacturing processes can result in improved reliability and yield so companies like Murata Power Solutions use burn-in data to drive a continuous improvement quality process.